1. Field of the Invention
The present invention relates to a backlight unit for use in display apparatus and also relates to a display apparatus having a backlight unit. More particularly, the present invention is concerned with the directivity of light.
2. Description of the Related Art
Liquid crystal display apparatus have been widely used in personal computers, liquid crystal display television systems, electronic organizers, cellular phones, and other terminal display devices. A backlight unit is provided behind a liquid crystal display panel of the liquid crystal display apparatus to make the displayed image appear bright and sharp. FIG. 18 in the accompanying drawings shows one type of conventional backlight unit structures.
A backlight unit 8 shown in FIG. 18 is provided directly below a liquid crystal display panel 1. The backlight unit 8 and the liquid crystal display panel 1 constitute a liquid crystal display apparatus 10. The backlight unit 8 comprises a lightguide plate 2, a light source 3 including LEDs disposed at one side surface of the lightguide plate 2, a light diffuser sheet 4 disposed directly above the lightguide plate 2, a pair of prism sheets 5 and 6 (first and second prism sheets) provided over the light diffuser sheet 4, and a reflector 7 provided directly below the lightguide plate 2. The first prism sheet 5 and the second prism sheet 6 have prisms each with a triangular sectional configuration and are stacked so that the ridges of their prisms project toward the liquid crystal display panel 1 and extend perpendicular to each other.
The backlight unit 8 having the above-described structure functions as follows. Light emitted from the light source 3, which includes LEDs, enters the lightguide plate 2 through the side surface thereof and exits from the upper surface of the lightguide plate 2 toward the light diffuser sheet 4 after the action of reflection by a prism surface (not shown) provided on the lower side of the lightguide plate 2. The light diffuser sheet 4 diffuses the incident light so that the quantity of light is uniformly distributed, and emits the diffused light toward the first and second prism sheets 5 and 6. The first and second prism sheets 5 and 6 direct the diffused light toward the liquid crystal display panel 1 at right angles thereto. Light emitting below the lightguide plate 2 through the prism surface on the lower side of the lightguide plate 2 is reflected by the reflector 7 to enter the lightguide plate 2 again. Thus, the backlight unit 8 is structured so that a maximum quantity of light from the light source 3 is effectively emitted and uniformly applied to the liquid crystal display panel 1, thereby brightly illuminating the displayed image on the liquid crystal display panel 1.
Japanese Patent Application Publication No. 2004-172003 discloses one of conventional techniques concerning backlight units. The structure shown in the Japanese Patent Application Publication has, as shown in FIG. 19, a light-directivity adjusting sheet 12 provided between a liquid crystal display unit 13 and a backlight unit 11, which includes a light source 11a, a lightguide 11b, a reflective sheet 11c, etc. The light-directivity adjusting sheet 12 is, as shown in FIG. 20, provided with quadrangular pyramids 12b. The backlight unit structure disclosed in the above-mentioned Japanese Patent Application Publication uses a single light-directivity adjusting sheet 12 provided with quadrangular pyramids 12b instead of using a stack of two prism sheets, thereby reducing the thickness of the backlight unit.
The above-described backlight structure for a liquid crystal display panel is arranged to emit light perpendicular to the display panel surface so that a bright display can be obtained on the display panel when viewed from a position directly in front of it.
FIG. 21 is an explanatory view schematically showing how a liquid crystal display panel is viewed. In FIG. 21, light (illuminating light) P is emitted from a backlight unit 15 with directivity in a direction perpendicular to the display panel surface of a liquid crystal display panel 16. The light P illuminates the display panel surface of the liquid crystal display panel 16 and passes therethrough to exit directly above (as seen in the figure) the display panel surface. When the display panel surface of the liquid crystal display panel 16 is viewed from a position A directly in front of the liquid crystal display panel 16, a sharp image will be seen on the brightly illuminated display panel surface.
However, when the display panel surface is viewed from either of positions B and C on opposite sides of the liquid crystal display panel 16, the displayed image on the display panel surface appears somewhat dark and lacks sharpness. Let's take up an on-board car navigation system. Generally, an on-board car navigation system is mounted in the vicinity of an approximately central position in the front of the car interior and thus positioned somewhat remote from and obliquely in front of the driver's and passenger's seats. When the display screen of the car navigation system is viewed from the driver's seat or the passenger's seat, the displayed image lacks sharpness and appears somewhat dark.